A typical ultrasound probe consists of three basic parts: (1) a transducer package; (2) a multi-wire coaxial cable connecting the transducer to the rest of the ultrasound system; and (3) other miscellaneous mechanical hardware such as the probe housing, potting material and electrical shielding. The transducer package is typically produced by stacking layers in sequence, as shown in FIG. 1.
First, a flexible printed circuit board 2 (hereinafter referred to the "transducer flex circuit"), having a plurality of conductive traces connected in common to an exposed bus, is bonded to a metal-coated rear face of a large piezoelectric ceramic block 4. The bus of the transducer flex circuit 2 is bonded and electrically coupled to the metal-coated rear face of the piezoelectric ceramic block. In addition, a conductive foil 10 is bonded to a metal-coated front face of the piezoelectric ceramic block to provide a ground path for the ground electrodes of the final transducer array. The conductive foil must be sufficiently thin to be acoustically transparent, that is, to allow ultrasound emitted from the front face of the piezoelectric ceramic block to pass through the foil without significant attenuation. The conductive foil extends beyond the area of the transducer array 4 and is connected to electrical ground.
Next, a first acoustic impedance matching layer 12 is bonded to the conductive foil 10. This acoustic impedance matching layer has an acoustic impedance less than that of the piezoelectric ceramic. Optionally, a second acoustic impedance matching layer 14 having an acoustic impedance less than that of the first acoustic impedance matching layer 12 is bonded to the front face of the first matching layer 14. The acoustic impedance matching layers transform the high acoustic impedance of the piezoelectric ceramic to the low acoustic impedance of the human body and water, thereby improving the coupling with the medium in which the emitted ultrasonic waves will propagate.
To fabricate a linear array of piezoelectric transducer elements, the top portion of this stack is then "diced" by sawing vertical cuts, i.e., kerfs, from the rear face of the stack to a depth sufficient to divide the piezoelectric ceramic block into a multiplicity of separate side-by-side transducer elements. The kerfs produced by this dicing operation are depicted in FIG. 2. During dicing, the bus of the transducer flex circuit 2 (not shown in FIG. 2) is cut to form separate terminals and the metal-coated rear and front faces of the piezoelectric ceramic block are cut to form separate signal and ground electrodes respectively. Electrically and acoustically isolated, the individual elements can now function independently in the array. Although the conductive foil (also not shown in FIG. 2) is also cut into parallel strips, these strips are connected in common to the conductive foil portion which extends beyond the transducer array 4, which conductive foil portion forms a bus which is connected to ground. Alternatively, the transducer flex circuit 2 can be formed with individual terminals instead of a bus and then bonded to the piezoelectric transducer array 4 after dicing.
The transducer stack also comprises a mass of suitable acoustical damping material having high acoustic losses. This backing layer 8 is coupled to the rear surface of the piezoelectric transducer elements to absorb ultrasonic waves that emerge from the back side of each element so that they will not be partially reflected and interfere with the ultrasonic waves propagating in the forward direction.
A known technique for electrically connecting the piezoelectric elements of a transducer stack to a multi-wire coaxial cable is by a flexible printed circuit board (PCB) having a plurality of etched conductive traces extending from a first terminal area to a second terminal area in which the conductive traces fan out, i.e., the terminals in the first terminal area have a linear pitch greater than the linear pitch of the terminals in the second terminal area. The terminals in the first terminal areas are respectively connected to the individual wires of the coaxial cable. The terminals in the second terminal areas are respectively connected to the signal electrodes of the individual piezoelectric transducer elements.
One approach for connecting a flexible PCB to a piezoelectric transducer array is a variation of a known high-density interconnect process originally developed for integrated circuit packaging and disclosed in U.S. Pat. No. 5,091,893. Using this technique, a flexible PCB can be fabricated with one end directly connected to a transducer array. To accomplish this, the transducer array is placed in a well formed in a frame with the metallized piezoceramic exposed. An insulating polyimide film is laminated to the surface of the metallized piezoceramic and the surrounding frame, creating a relatively flat surface. A computer-controlled laser then ablates holes in the polyimide layer down to the metal electrode atop the ceramic. A metal layer is applied over the film and follows the hole contour, thereby making electrical contact with the metal electrodes on the ceramic. Conventional photolithographic techniques (25 .mu.m lines and spaces are typical) are used to pattern the metal, thus creating lines from each transducer element to a fanout pattern. The process can be repeated to produce multilayered structures. Excess polyimide can be removed to provide a good acoustic contact of the backing to the ceramic element.
The above-described high-density interconnect system allows the transducer designer to interconnect elements at a considerably higher density than standard manual soldering or flexible PCB technology. This is particularly useful when the transducer design requires fine-pitch, high-frequency operation.
As the system demands on element count in these devices increase, the requirements for making electrical connection to new complex transducer geometries approach the point of being insurmountable. One of the most difficult tasks is the process of connecting signal ground to the front face of the transducer piezoelectric ceramic.
In particular, the density requirements of the transducer array are challenged by the transducers needed for multi-dimensional imaging. These transducers require elements in two dimensions, instead of the one-dimensional designs required by conventional imaging apparatus. When the electrical interconnect becomes two-dimensional, however, the designer is faced with the challenge of providing an electrical interconnect for transducer elements which are no longer accessible from the sides of the array, which is a feature common to most conventional transducer designs. In order to connect the internal elements, complicated methods have been proposed and developed.